physiology of hypothyroidism Flashcards
(33 cards)
where is most of T3 produced
peripheral tissues
what are the types of hypothyroidism and where do they each originate
- primary: thyroid disease
- secondary: pituitary origin
- tertiary: hypothalamus
discuss primary hypothyroidism
- lack of functional thyroid tissue
- most common form
- can be acquired, iatrogenic or congenital
- acquired usually due to: lyphocytic thyroiditis, idiopathic follicular atrophy or secondary to neoplasia
- iatrogenic usually due to surgery, radioactive iodine therapy or anti-thyroid medications
- congenital usually due to cretinism (early death), thyroid gland agenesis or hysgenesis, thyroid peroxidase deficicy or dificient dietary iodine/ingestion of goitrogens
discuss secondary hypothyroidism
- impaired secretion of TSH from pituitary
- uncommon
- acquired usually due to neoplasia, pituitary suppression, illness or malnutrition
- congenital is rare but usually due to cystic rathke’s pouch or accompanied by other pituitary hormone deficiencies
discuss tertiary hypothyroidism
- lack of TRH in the hypothalamic supraoptic and paraventricular nuclei
- rarely documented in the dog
what is the most common cause of hypothyroidism in dogs
lymphocytic thyroiditis
or
idiopathic follicular atrophy
what is the signalment of canine hypothyroidism
- breeds: dobermans, goldens, cocker spaniels, irish setters, terrier
- no sex predisposition
- peak incidence 4-6 years
list clinical signs of canine hypothyroidism
- vague, diffuse and gradual onset
- not pathognomic for the disease
- most common signs are dermatologic or metabolic
- appear dull
- lethargy
- exercise intolerance
- obesity without history of polyphagia
- cold intolerance
- skin changes
how does hypothyroidism affect development
- reduced development and maturation of brain cells in fetus and young
- important to have thyroid during fetal period and first few months after bitth
how does hypothyroidism affect growth
- growth retardation
- smaller
- shorter bones
- delayed closure of physes (growth plates)
what are the metabolic actions of thyroid hormones
- increase carbohydrate metabolism
- increase fat metabolism
- increase basal metabolic rate
- decrease body weight
how does hypothyroidism affect carbohydrate metabolism
- thyroid hormone stimulates glucose metabolism, increasing glucose uptake, insulin sensitivity, insulin secretion, glycolysis and gluconeogenesis
- therefore, hyporhtyroidism slows carbohydrate metabolism leading to weight gain
how does hypothyroidism affect metabolism of fats
- thryoid hormone enhances fat metabolism
- mobolizes lipids from adipose stores
- accelerates oxidation of lipids to produce energy
- therefore hypothyroidism slows fat metabolism leading to weight gain
how does hypothyroidism affect basal metabolic rate
- increased BMR in all tissues except the brain, gonads and spleen
- leads to increased heat production and oxygen consumption
- hypothyroidism leads to lower BMR (50% of normal) leading to weight gain
how does hypothyroidism affect musculoskeletal
- reduces muscle tone and changes in fibre type
how does hypothyroidism affect cardiovascular system
normal physiological effects on the heart:
- increase blood flow and cardiac output
- increase heart rate
- increase contractility
hypothyroidism = bradycardia, weak apex beat, electrocardiogram = low voltage complexes, echo = decreased fractional shortening
how does hypothyroidism affect the respiratory system
- resp system indirectly affected by T3 and T4
- normal action = increase in basal metabolic rate (caused by increased demand for oxygen and excretion of CO2)
how does hypothyroidism affect the nervous system
- thyroid hormones normally needed for normal development, enhances SNS and optimal nerve conduction
- in hypo, neurological abnormalities such as peripheral neuropathy (knuckling, paresis, hearing impairment, slower reflexes) and myopathy (paresis and slow gait)
how does hypothyroidism affect the CNS
- mentally slower (hard to quantify in animals)
- lethargic and require more sleep
how does hypothyroidism affect the reproductive system
- reduced sexual drive
- infertility
- shortened oestrus
- prolonged eostrual bleeding
- prolonged anoestrus
- testicular atrophy
- reduced spem production
how does hypothyroidism affect the GI system
- thyroid hormone increases appetite and feed intake, increases secretion of pancreatic enzymes and increases motility in normal animals
- leads to consipation in hypothyroid
how does hypothyroidism affect the integument
- thyroid growth initiates and maintains anagen phase normally
- in hypo, get arrests of hair growth, hair is retained in telogen and alopecia or failure to regrow hair after clipping
what do canine hypothyroid skin changes appear as
- bilateral symmetrical lopecia (esp in areas of wear or pressure points, not pruritic unless pyoderm underlying)
- seborrhea, lichenification and comedones
- hyperpigmentation on alopecic areas
- recurrent infections (otitis externa and pyoderma)
- rat tail of bald bridge of nose
what is myxedema and how does it relate to hypothyroidism in dogs
- excess mucopolysaccharides and hyaluronic acid in dermis
- tragic facial expression
